1. Field of the Invention
The present invention demonstrates the structure of a new, simple and small-size hydrogen-sensitive palladium (Pd) membrane/semiconductor Schottky diode sensor and the method of fabricating it.
2. Description of the Prior Art
Recently gas sensors have been widely used in controlling the industrial process, prevention of the environmental pollution and medical diagnostics. People have paid attention to the detection of the concentration of the hydrogen gas and conducted research due to its danger of explosion. The series of sensors related to Pd membrane have attracted much attention among all kinds of hydrogen-sensitive sensors. The main reason is that palladium metal is an active catalyst. It will dissociate the hydrogen molecules on the Pd membrane surface into hydrogen atoms. Some of the hydrogen atoms diffuse through the palladium metal layer and adheres to the palladium metal-semiconductor or palladium metal-insulating layer interface. The palladium hydride may effectively lower the work function of Pd metal. The lowering of work function results in the modification of the measured current-voltage characteristics. I. Lundstrom et al. have proposed MOS field effect transistor with Pd gate on silicon substrate in J. Appl. Phys., Vol. 46, pp. 3876 (1975). The sensor of this transistor structure is quite sensitive to hydrogen gas, but the difficulty in processing and the cost of which is high since the structure of transistor is more complicated and also need one layer of good quality SiO.sub.2 insulating layer passivated on the top of the silicon substrate. Another simpler structure is to evaporate palladium metal on a semiconductor substrate to form a Schottky barrier. During the hydride formation process, the work function of Pd metal is lowered, which results in the reduction of Schottky barrier height to modify the output current characteristics of the studied device. Earlier Schottky diode hydrogen-sensitive sensor is fabricated by evaporating Pd metal on the substrate of II-VI semiconductor, e.g. the Pd--CdS structure reported by M. C. Steele et al. in Appl. Phys. Lett., Vol. 28, pp. 687(1976) and the Pd--ZnO structure reported by K. Ito in Surf. Sci. Vol. 86, pp. 345. These Schottky diode devices can be used as for hydrogen-sensitive sensor. II-VI semiconductor substrate is used since the surface state density of IV and III-V semiconductor is higher so that the barrier height and the metal workfunction is not so closely related.
In this invention, a new hydrogen-sensitive Schottky diode sensor is proposed. The Schottky barrier is formed by evaporating palladium metal on the surface of III-V semiconductor (GaAs). To reduce the surface state density of the semiconductor, first, a high quality undoped GaAs buffer layer and an n-type GaAs epitaxial layer is grown by molecular beam epitaxy (MBE) on a semi-insulated GaAs substrate. Then a thin Pd membrane is evaporated on the surface of the n-type GaAs epitaxial layer by the vacuum evaporation technique of round metal mask. Since the surface state density of IV and III-V semiconductor is lowered, the work function of Pd metal will be lowered during the hydride formation process, which results in the reduction of Schottky barrier height to modify the output current characteristics of the studied device. Therefore the goal of hydrogen-sensitive sensor can be achieved. The present invention has the advantage of simple device structure & process, small size, so it has great potential in practical applications. Furthermore, the process technology of III-V semiconductor is far more sophisticated and stable than that of II-VI. Therefore, the sensor of the present invention can be combined with the other III-V components (e.g. high speed transistor, laser . . . ) to form excellent and versatile intelligent optoelectronic integrated sensor circuit.